This invention relates to power combining radio frequency signals, and more particularly, this invention relates to a power combining network for combining radio frequency signals using microstrip and waveguide circuits.
Power combining techniques for radio frequency signals, including millimeter wavelength signals, have been accomplished in either a waveguide circuit or in a microstrip circuit. For example, prior art waveguide combining has been accomplished by feeding two or more signals in phase into a waveguide combiner. Although this type of power combining is efficient, the summing network is generally bulky and requires very high precision components. Microstrip power combining circuits have been accomplished by summing signals using a hybrid combiner circuit or a Wilkinson power summer circuit as known to those skilled in the art. This type of power combining circuit is more simple to implement in practice, but generally has higher losses.
FIG. 1 illustrates a typical waveguide combiner 20, widely available in the industry, and traditionally used to combine radio frequency signals from two sources of RF power. The combiner 20 can be formed from different materials as known to those skilled in the art, and generally has two input ports 22 that are bolted or fastened by other techniques to respective waveguide sources. The signals combine and are summed at the output port 24. This combiner 20 provides a reliable method of adding radio frequency energy, but requires careful phase matching of two radio frequency inputs and precisely control over the length of the two waveguide sides 26. The precision requirements for this waveguide and the requirement for a metal coating on the inside surface of the waveguide to achieve low losses results in relatively expensive devices. Also, this waveguide combiner is usually bulky, as illustrated, and occupies a significant amount of space.
FIGS. 2-4 show typical microstrip power combiners formed from microstrip transmission lines. These type of combiners are widely used in the industry for combining radio frequency power in microstrip circuits. There are primarily two types of microstrip combiners, using Wilkinson and hybrid circuits, as shown in the schematic circuit diagrams of FIGS. 2 and 3, respectively. The Wilkinson combiner 30 shown in FIG. 2 is a reflective combiner and includes two inputs 32, an output 34, and the Wilkinson circuit 36 that has a resistor for circuit balance as known to those skilled in the art. The hybrid combiner 40 shown in FIG. 3 is absorptive and includes two inputs 42, an output 44, and load resistor 46, forming a four port hybrid combiner. FIG. 4 illustrates a plan view showing the microstrip transmission lines 48 forming the circuit. In the hybrid combiner 40, the load resistor 46 absorbs any reflected energy because of mismatch. Typically, the three decibel (dB) Wilkinson combiner 30 results in 0.5 dB loss, while the hybrid combiner 40 results in 0.8 dB losses. These combiners provide a reliable method of RF energy summing and can be used in a very small space.
Other examples of various types of combiners and different RF coupling systems are disclosed in U.S. Pat. Nos. 4,761,654; 4,825,175; 4,870,375; 4,943,809; 5,136,304; 5,214,394; and 5,329,248.
As is also known to those skilled in the art, in a waveguide-to-coaxial line connector, a maximum energy field is in the center of the waveguide. An extension of a center conductor can be located at the point of a maximum energy field and act as an antenna to couple energy from a coaxial line into a waveguide. Coupling from a coaxial line to a waveguide could be achieved by using a loop, which couples two magnetic fields. In a prior art waveguide circuit using stripline or microstrip, the center conductor of a stripline can be extended into a waveguide forming a probe (or launcher). By increasing the width of a center conductor at the end of a probe, bandwidth can be improved. Also, the conductor and substrate of a microstrip circuit, but not a ground plane, can be extended directly into a guide.
In a prior art coaxial line circuit using a microstrip connection, the center conductor of a coaxial line can be pressed against or soldered to a conductor of a microstrip. The outer conductor of a coaxial line can be grounded to a microstrip ground plane. The microstrip substrate thickness could be as little as 0.010 inch for frequencies above 15 GHz, and usually requires decreasing the diameter of the coaxial line. In yet other types of systems, various directional couplers have waveguides that are located side-by-side or parallel to each other, or crossing each other. Stripline and microstrip couplers can have main transmission lines in close proximity to secondary lines Although these examples can provide some power combining and coupling, they are not useful for combining two or more sources of radio frequency energy in a microstrip-to-waveguide transition with low losses or small xe2x80x9creal estatexe2x80x9d at an efficient rate at low power loss.
It is therefore an object of the present invention to provide a microstrip-to-waveguide and a coaxial-to-waveguide power combiners that overcome the disadvantages of the prior art power combiners identified above and has low losses, small xe2x80x9creal estate,xe2x80x9d and is power efficient.
The present invention is advantageous and power combines radio frequency signals using a combination of microstrip and waveguide circuit techniques that result in very low losses. The combining network is compact and can be used at a low cost. In the present invention, two or more sources of radio frequency energy can be combined in a microstrip-to-waveguide transition resulting in low losses. Also, two or more sources of radio frequency energy in a microstrip-to-waveguide transition are combined and are not as sensitive to phase mismatch between the radio frequency sources as other power combine methods. The power combining is achieved efficiently at a low cost and is implemented in compact spaces. The method and apparatus of the present invention allows radio frequency power combining that can be implemented at any frequency where energy can be transferred over a waveguide.
In accordance with one aspect of the present invention, the microstrip-to-waveguide power combiner includes a dielectric substrate and at least two microstrip transmission lines formed thereon in which amplified radio frequency signals are transmitted. The at least two microstrip transmission lines terminate in microstrip launchers (probes) at a microstrip-to-waveguide transition. A waveguide opening is positioned at the transition. The waveguide back-short is positioned opposite the waveguide opening at the transition. Isolation/ground vias are formed within the dielectric substrate and positioned around the transition to isolate the transition and provide a ground well. The radio frequency signals can be millimeter wavelength radio frequency signals.
In yet another aspect of the present invention, a metallic plate supports the dielectric substrate. A back-short cavity is formed within the metallic plate at the transition to form the waveguide back-short. This back-short cavity has a depth ranging from about 25 to about 60 mils and its overall dimensions are about the size of the waveguide opening. The back-short is positioned for reflecting energy into the waveguide opening.
In yet another aspect of the present invention, each microstrip transmission line has a power amplifier associated therewith and supported by the dielectric substrate. The phase of each power amplifier is adjusted based on the location of microstrip launchers or probes at the transition. The number of microstrip launchers, in one aspect of the invention, can be either two or four and the respective phase of the power amplifiers is 180 degrees apart for two opposed microstrip launchers or 90 degrees apart for four microstrip launchers when positioned at 90 degree angles to each other. The power amplifiers comprise microwave monolithic integrated circuits (MMIC) in one aspect of the invention.
A method aspect of the present invention is also disclosed for power combining radio frequency signals by combining two or more amplified radio frequency signals at a microstrip-to-waveguide transition that is formed from a dielectric substrate having at least two microstrip transmission lines thereon in which radio frequency signals are transmitted. The transition includes a waveguide opening and a waveguide back-short positioned opposite the waveguide opening. Each microstrip transmission line has a microstrip launcher or probe extending into the transition. Isolation vias are formed within the dielectric substrate around the transition and isolate the transition and provide a ground well around the transition.